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Science of Backpressure

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Old 09-17-2012, 06:14 PM
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Default Science of Backpressure

Im always looking around on this info but I'm always looking for more perspectives and experiences.

So when I first started messing around with my car I didn't know too much- still don't. But I would ask around and then buy a bolt on mod and learn how to work with my car. So I learned how to take things apart and replace them. However I find- that matches up with my reading- that there are sacrifices to certain bolt on mods.

Correct me if I'm wrong cuz I wanna learn, but what I see is that with increasing a cars air flow it eventually loses low end torque for the high end torque. So I'm trying to find out what the right balance is. I'm doing all this to my current car so I can get a better car and do it all better at least the second time around.

Currently with my bolt on mods: CAI, 421 Header, 2.5 dia. exhaust, and f23 intake manifold, it breathes well and has more of a kick at higher speeds but I feel like my low end torque and even city MPGs have suffered. On the highway the car does great. I get up to 36 MPG at 70 mph.

So whats the science behind it if what I'm seeing is true? Or is there a placebo affect going on?
 
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Old 09-17-2012, 06:40 PM
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You've got to pay Paul if you borrow from Peter; changing the flow and resonance characteristics of your engine will very often do just what you describe, hurt mid range performance while providing a small bump in top end power.

I've seen what you've described many times over, sounds like it's true.
 
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Old 09-17-2012, 09:05 PM
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Ok. Thanks, I asked in another forum and was told to look up volumetric efficiency. So Ill look more into that. I'm glad to see that I'm at least headed in a direction thats been dealt with and I'm not messing up my car. What solution is there to make up for my lost low end torque?
I'm looking into better fuels, tricking the ECU or even restricting the exhaust all the time with a silencer. I drive around town a lot. So its a lot of stop and go. Is a silencer my best option?
 
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Old 09-18-2012, 07:00 AM
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Originally Posted by RobinsonRicer View Post
Ok. Thanks, I asked in another forum and was told to look up volumetric efficiency. So Ill look more into that. I'm glad to see that I'm at least headed in a direction thats been dealt with and I'm not messing up my car. What solution is there to make up for my lost low end torque?
I'm looking into better fuels, tricking the ECU or even restricting the exhaust all the time with a silencer. I drive around town a lot. So its a lot of stop and go. Is a silencer my best option?
From my perspective, a bone stock factory setup is typically the best for low to mid range torque; the only proven way to increase those numbers is to add more cylinders or add forced induction neither of which are cheap in any incarnation.

FWIW, supercharging your motor will provide a very sizable bump in low end torque, however, superchargers typically become less effective as engine RPM increases. Turbocharging on the other hand, typically provides very little improvement in low end torque, the flip side of course is that as RPMs come up, to too does the rise in mid range power.

As I see it, this is one of those pick your poison scenarios, you can:
  1. Sell your car and buy an otherwise identical V6 Accord, the downside is that you will lose your manual transmission.
  2. Add a supercharger to your car and get a very nice improvement in low-end torque, however, a supercharger can rather dramatically reduce fuel economy and won't provide much (if any) of an improvement in output up near red-line.
  3. Add a turbocharger to your car and get a nice improvement in mid-range torque (above say 1800-2500 RPMs, depending upon the size of the turbocharger you use).
 
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Old 09-18-2012, 03:04 PM
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Ok. Thanks. I was never really planning on doing any dramatic changes to my car. I'm just messing around with it and getting a feel for what I can do on a good car. I hear the v6 accords from the 5th gens arent that great though. 170 hp. Im still thinking about a TSX for my next car and to put a lot more money into that. Like a turbo and new cam, reflash, all that.
I find with a silencer (restricting exhaust) I get a bit more low end torque. I guess thats my best option for around the city.
Would it make sense that with less low end toque the car works harder and uses more gas to accelerate from start thus using more gas than when accelerating at higher speeds? Would that explain why the gap in fuel savings from city to highway is so big?
 
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Old 09-20-2012, 06:33 PM
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Volumetric efficiency might be an acedemic exercise for you. Not to discourage you from acedemics...

"Backpressure" isn't as simple as most people think. It's an acoustics problem, with pulses reflecting back & forth from branches in the exhaust system. That's the science behind choosing the length of the exhaust runners. How far back are the 4-into-2 branches. How much further yet is the 2-into-1 branch. Or maybe you choose a 4-into-1 header... Speed of sound in that pipe, with hot exhaust gas, isn't your normal 1100 ft/sec either. Besides being superimposed on the bulk exhaust flow. It's not like flowing water in a garden hose.
 
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Old 09-20-2012, 07:39 PM
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interesting. I found an explanation that I understood. Lemme know if its right. Basically it used the garden hose. The smaller the exhaust the faster the exhaust comes out. In those pulses theres low pressure behind it which helps the next pulse through. Bigger the exhaust the faster the velocity must be to keep that up. Is that the basics of it?
And should I invest in a better header? I bought one that came with the catback exhaust but am wondering about the quality. My car drives fine as far as the exhaust goes but I wonder if I got a decent header and front pipe it would make a difference.
 
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Old 09-25-2012, 09:44 AM
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It's pretty bogus to relate this to a garden hose. People say that if you pinch off the end of the hose you get faster flow. MISLEADING! You get higher velocity, but you also get more restriction and LOWER flow rate (gallons per minute). The biggest problem is most people don't have physical experience with pulsating flow in a garden hose. Even a household power-washer (driven by a piston pump) has smoothed out the flow before it reaches the nozzle, so even that's almost steady-flow.

Yes, each pulse is followed by low-pressure which helps the next next pulse. The rest of the explanation is really oversimplified. But without the proper math for the acoustics problem, there might not be a better explanation. You need the right conditions to prevent the high-pressure pulse from reflecting back and OPPOSING the next pulse.
 
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Old 09-25-2012, 05:48 PM
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Yeah, I found that explanation to be the one that helped the light go off in my head. And now I understand it well. As for the acoustics, I gotta research that more. But the high and low pressure with the higher velocity makes sense now.
 
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